The present invention relates to scanning probe microscopes and methods for using the same. In particular, the present invention relates to scanning probe microscopes and methods for using the same to detect the transfer of ultrasmall amounts of charge or current to or from a conducting surface under analysis and additionally measuring the electronic response of that surface to the transferred charge.
Scanning probe microscopes (SPMs) include several related technologies for imaging and measuring surfaces on a fine scale, down to the level of molecules and even single atoms. SPM techniques have the ability to operate on a scale from many microns down to sub-nanometers and can image individual atoms and molecules.
SPM technologies share the concept of scanning an extremely sharp tip (up to 50 nm radius of curvature) across the object surface. The tip is often mounted on a flexible cantilever, allowing the tip to move and follow the surface profile. When the tip moves in proximity to the investigated object, various forces of interaction between the tip and the surface influence the movement of the cantilever and are detected by selective sensors. Various other interactions can be measured and studied depending on the type of SPM used.
There are numerous types of SPM technologies, including atomic force microscope (AFM), scanning tunneling microscope (STM), lateral force microscope (LFM), magnetic force microscope (MFM), electrostatic force microscope (EFM), and scanning thermal microscope (SThM). STMs measure a weak electrical current flowing between the tip and the sample. STMs rely on the electrical conductivity of the sample, so features on the sample surface must be electrically conductive to some degree. STM systems measure the quantum tunneling current between a wire or metal-coated silicon tip and the object surface. An electronic feedback system maintains a constant current by positioning the tip at a substantially constant height above the surface.
The intense interest in understanding and utilizing the properties of atomic scale structure has motivated a significant effort to develop instrumentation for the ultrasmall domain. STMs, in particular, have been developed to provide characterization, manipulation and modification in this domain. The unprecedented atomic resolution achieved by STM has provided a direct method for visualizing and manipulating atoms and atomic scale structures. Additionally, the STM has been used to characterize the electronic properties of atomic scale surface structure of the sample being analyzed via stanning tunneling spectroscopy (STS). These abilities make it extremely useful tool for exploring the xe2x80x9cnanoworld.xe2x80x9d
The capabilities of the STM, however, are not limitless. In particular, the STM is limited to imaging structures having sufficient conductivity to provide a measurable current in a reasonable measurement time. Most STM measurements are performed with currents greater than a picoampere (10xe2x88x9212 ampere). Serious efforts to optimize current detection techniques have pushed the STM current detection limits into the 0.1 picoampere (10xe2x88x9213 ampere) range, and even lower currents can be measured (10xe2x88x9214 ampere range) if integration times are increased. However, imaging rapidly becomes very tedious under such conditions.
If the current detection sensitivity of the STM could be improved from the 100 femtoampere range (10xe2x88x9213 ampere≈106 electrons/sec) to the attoampere range (10xe2x88x9218 ampere≈10 electrons/sec) previously inaccessible surfaces and applications for the STM would emerge. For example, imaging many biomolecules by STM has been significantly limited by the poor conductivity of these molecules. Imaging these molecules at currents of about 103 to about 106 times smaller would provide several benefits, namely allowing imaging at larger tip/sample separations (larger tunneling gaps) that would reduce the tip-molecule interaction forces which have plagued much of the STM work on weakly adsorbed molecules. As well, molecules that appear as xe2x80x9cinsulatingxe2x80x9d at currents in the 10xe2x88x9213 ampere range may appear as xe2x80x9cconductingxe2x80x9d for currents in the 10xe2x88x9218 ampere range.
Due to the limitations of STM, many have turned to AFM to image insulating structures and molecules. While the AFM overcomes the need for electrical conductivity, true atomic spatial resolution by AFM is especially difficult on soft surfaces. As well, AFM devices have not been used to measure the tunneling of electron charges to or from a surface. The AFM also lacks the ability to directly measure the electronic properties of a surface.
If the current detection sensitivity of the STM could be improved to allow detecton of single electrons, then tunneling to or from insulating or localized surface states would be possible. Such sensitivity would also open up a new class of insulating materials that could be studied on the atomic scale. STMs with ultrahigh current sensitivity (i.e., attoamperes) might measure the charge transfer through thin oxides on semiconductors, between quantum dots and clusters, or to single electron devices. Many of these opportunities, as well as others, could be usefully explored by STMs at these ultrasmall currents.
The present invention develops a new type of SPM, a scanning tunneling charge transfer microscope (STCTM). The STCTM is capable of first, detecting the transfer of an ultrasmall amount of charge (single electrons) or ultrasmall current (attoampere) into or out from a surface with atomic resolution, second, detecting the energy at which the charge is transferred, and third, measuring the electronic response of that surface to the transferred charge. These capabilities can be achieved by appropriately combining the virtues of the STM and a modified EFM. The STM capability provides the atomic resolution for the charge transfer via tunneling, while the modified EFM capability provides the sub-electronic charge sensitivity for the current and charge transfer detection. The STCTM, with sensitivity many orders of magnitude better than with SPM technology currently available, can be used to characterize the properties of molecules, ultrathin oxides, insulator surfaces, and clusters on insulators, among others, with atomic spatial resolution.